This invention relates to an improved method and apparatus for electrostatically charging electrets and, more particularly, the invention is concerned with providing a technique for charging TEP Teflon tape by drawing past a wetted sponge containing a dielectric fluid mixture of methyl alcohol, ethyl alcohol and acetone. By endosmosis, a bead of fluid forms between the sponge and the FEP Teflon tape producing an evenly distributed over the tape, and thereafter is dried.
In the design of a reliable perimeter intrusion detection system, the electret tape concept was developed. An active ultrasonic doppler detection scheme utilizes a long, thin multi-layered tape containing an electret layer as the radiating and receiving transducer. This transducer is essentially an elongated electret microphone which functions as a radiator as well. The highly directional ultrasonic CW beam radiated by the tape uniformly all along its length illuminates any object crossing it. This action causes a reflected doppler-shifted ultrasonic signal that is received by the tape and separated from the driving signal by special hybrid electronic circuitry. A special processor then operates on the received signal and generates an appropriate alarm.
A large variety of techniques have been utilized in an attempt to produce electrets with increased and controlled electret strength with improved uniformity of interlayer spacings while still capable of being easily adaptable to large scale manufacturing. These techniques include irradiation of dielectrics usually in the presence of an electric field, electromagnetic radiation ranging in wavelength from thermal to gamma regions and, also, bombardment of dielectrics by charged-particle beams. None of these techniques was entirely satisfactory for a number of reasons including, poor uniformity, not easily repeatable, difficult of controlling, time consuming, etc.
Another charging process called "the liquid contact" technique has been employed using ethyl alcohol, acetone and water as the liquid contacts. A film of the contact liquid was applied to the upper surface of the polymer by the charging electrode. The resulting electrets lacked uniformity and the electret strengths varied considerably among the samples even though what seemed to be identical treatments were used. Also, over a long length of tape requiring continuous operation, the previous liquid-contact technique of using a sponge saturated with ethyl alcohol as the positive electrode was not satisfactory because it could not be adapted to produce a uniform charge over the entire length. A new design capable of allowing continuous charging of long samples to make long tapes of greater uniformity to a higher level than obtainable by previously known procedures is required.